In recent years serious concern for the protection of groundwater from various pollutants has risen. It has been found that contaminants may enter groundwater from various surface locations rendering groundwater unsuitable for drinking or other uses. Among the potential sources of groundwater pollution are leaking underground tanks and, specifically, underground hydrocarbon storage tanks. In many cases, gasoline storage tanks associated with service stations have been found to leak gasoline to the water table.
Light hydrocarbons (e.g., gasoline) lost from refinery tanks, pipelines and service stations gravitates downward to settle on top of the water table in the shape of a plume. In the center of the plume, the hydrocarbon thickness could range up to a couple of feet. Recovery of spilled gasoline is usually effected by pumping the fluid back up to the surface, for example, see U.S. Pat. Nos. 4,630,677; 4,497,370; and 4,273,650.
It is necessary to obtain an estimate of the total quantity of gasoline remaining in the plume at any given time to estimate the volume of hydrocarbon spilled as well as to assess the progress of the cleanup. This can be done by determining the thickness of the plume in several locations over its areal extent. The current technique for doing this is to drill an open well through the plume, and let the water and the gasoline flow into the well and stand to their hydrostatic equilibrium levels. The depths of these fluids and thickness of hydrocarbon in the well can then be measured by simple methods readily known to one skilled in the art (see U.S. Pat. No. 4,213,340).
This wellbore is not representative of the fluid/solid matrix in the formation, however, and the thickness of the free-standing column of gasoline in the open well is generally greater than the actual formation thickness of the spilled fluid [J. Van Dam, "The Migration of Hydrocarbons in a Water-Bearing Stratum", Schwille, F., "Petroleum Contamination of the Subsoil--A Hydrological Problem", both in "The Joint Problems of Oil and Water Industries, Symposium held at Brighten, England (1967), all incorporated by reference herein].
No good means of quantifying this discrepancy exist, but it is believed that the two numbers could differ by a factor of as much as 2 to 4. Using the thickness of the hydrocarbon layer in the wellbore to estimate the amount of hydrocarbon spilled would cause a greatly exaggerated estimate of the amount of gasoline spilled.
It is, therefore, desirable to devise a method and means of accurately determining the actual thickness of hydrocarbon in a subsurface formation.